1. Field of the Invention
This invention relates to a magnetite-iron based composite powder, a magnetite-iron based composite powder mixture and a method for producing the magnetite-iron based composite powder mixture. The invention also relates to a method for remedying polluted soil, water or gases with the aid of the reducing activity of the magnetite-iron based composite powder and to an electromagnetic wave absorber and other magnetic material using the magnetism of the magnetite-iron based composite powder.
2. Description of the Related Art
Iron powders are used as materials for powder metallurgy for fabricating mechanical parts and magnetic parts such as magnetic powder cores and electromagnetic wave absorbers. In addition, iron powder is used as a powder in catalysts, food additives, antioxidants and carriers for copier toner and for remedying soil and other media. Conventional techniques and problems thereof will be illustrated below specifically regarding methods for remedying soil and other media using a fine iron powder as a reducing agent and electromagnetic wave absorbers using the magnetic properties of the iron powder.
Remediation of Soil and Other Media Using Iron Powder
Methods for remedying soil or groundwater polluted by an organic halogen compound can roughly be classified as (1) a method in which polluted soil or groundwater is decomposed in situ (an in situ decomposition method), (2) a method for treating gases in the polluted soil or polluted groundwater after pumping from the ground (treatment after in situ extraction), and (3) a method for treating the polluted soil after excavation (excavation treatment).
Methods using an iron powder as a reducing agent for decontamination of harmful organic halogen compounds by dehalogenation have been proposed. For example, PCT Japanese Translation Patent Publication No. 5-501520 and Japanese Unexamined Patent Application Publications No. 10-263522 each propose a method for remedying soil and soil moisture by forming a dispersed iron powder layer in the soil followed by bringing groundwater into contact with the layer to thereby decompose organic halogen compounds. Japanese Unexamined Patent Application Publication No. 11-235577 also proposes a method for remedying soil by adding iron powder to and mixing with the soil (excavated or not) to thereby decompose organic chlorine compounds.
The iron powder used in the former method (PCT Japanese Translation Patent Publication No. 5-501520 and Japanese Unexamined Patent Application Publications No. 10-263522) is, for example, scrap iron produced in the cutting process of iron. It is hard to control the composition and structure of the iron powder to be suitable as a reducing agent for organic halogen compounds. As a result, the iron powder exhibits insufficient effects. In addition, the former two publications mention that iron oxides are formed on the surfaces of iron particles by reaction with oxygen in the soil to thereby deteriorate reduction power of the iron powder. As a countermeasure to this problem, the publications also propose deoxygenation of soil in the vicinity of the iron powder by allowing a reducing substance to disperse in the soil. This means that the iron powder used in this method does not have sufficient durability in its reduction power.
The latter method (Japanese Unexamined Patent Application Publication No. 11-235577) proposes an iron powder containing equal to or more than 0.1% by weight of carbon and having a specific surface area of equal to or more than 500 cm2/g. This iron powder comprises sponge like particles having a pearlite texture as a structure with a particle size distribution that allows equal to or more than 50% by weight of the total powder to pass through a 150 xcexcm sieve. However, even this configuration may not effectively dehalogenate such organic halogen compounds.
Japanese Unexamined Patent Application Publication No. 12-80401 proposes an iron powder containing 0.020 to 0.5% by weight of phosphorus, sulfur or boron as iron powder that can effectively remove phosphor compounds in drainage. The iron powder contains specific trace elements and the objective effect thereof is to accelerate decontamination of phosphor in the drainage by increasing the dissolving speed of the iron into the drainage. Specifically, according to the mechanism of the aforementioned iron powder, a compound which hardly dissolves and has a low solubility product constant, such as iron phosphate, is formed between the dissolved iron and phosphor in the drainage to remove phosphor from the drainage by precipitation. This technology is fundamentally different from the technology for reductive decomposition of harmful substances on the surface of iron according to this invention.
Japanese Unexamined Patent Application Publication No. 2000-5740 proposes an iron powder containing 0.1 to 10% by weight of copper as an iron powder that can efficiently remove organic chlorine compounds from soil and/or groundwater. However, copper itself is a harmful metal with a danger of causing secondary pollution.
In addition, all of the aforementioned iron powders mainly contain iron having a valency of zero (Fe0) and thereby exhibit insufficient decomposition power for organic halogen compounds.
An iron powder having an average primary particle size of less than 1 xcexcm has not been readily available, and those having a large average primary particle size of about 80 xcexcm have been used for the above application. However, such iron powders having a large particle size cannot sufficiently be dispersed into the soil or groundwater, have a small specific surface area and thereby cannot sufficiently decompose the organic halogen compounds with efficiency. Even if such fine iron powder particles can be obtained, their reduction power may be rapidly consumed.
Application of Iron Powder to Magnetic Materials
A carbonyl ion powder obtained by reduction of carbonyl iron, and an acicular iron powder obtained by reduction of goethite iron (acicular iron oxide) are widely used as magnetic materials for use in electronic equipment and communication equipment. However, a demand has been made on magnetic materials that can exhibit their functions in the high frequency regions as a result of recent advances in electronic and communication equipment.
The carbonyl iron powder comprises relatively large particles with a particle size of about several micrometers and its permeability decreases with increasing frequency. The carbonyl iron powder cannot, therefore, be used in a noise filter or an electromagnetic wave absorber in the high frequency regions with a frequency on the order of gigahertz (Ghz).
The acicular iron powder comprises relatively small particles with a particle size of about 0.1 xcexcm, but such constitutive small particles easily aggregate to thereby form an aggregate having a relatively large particle size. Accordingly, the acicular iron powder is also limited in its application as in the carbonyl iron powder.
In addition, the material carbonyl iron and goethite iron cannot stably be manufactured in a high volume and are expensive, thereby causing increased manufacturing costs.
As such an electromagnetic wave absorber for use in the high frequency regions on the order of gigahertz, a sheet prepared by molding a mixture of a flat powder and a resin is used (e.g., Yasuo Hashimoto: xe2x80x9cCeramicsxe2x80x9d vol. 35, No. 10 (2000), p. 857-862). The flat powder used herein is prepared by processing a Fexe2x80x94Si alloy powder, sendust powder or stainless steel powder into a flat powder. However, such flat powders require expensive material powder and expensive treatments for pressing the material powders, thereby causing increased manufacturing costs of the product electromagnetic wave absorber.
Japanese Unexamined Patent Application Publication No. 1-136910 proposes a method of manufacturing a reduced iron including fewer impurities and oxide films by reducing an iron oxide obtained from a pickling device for hoop steel. However, fine particles of the resulting pure iron with a particle size of from 0.1 to 3.0 xcexcm are immediately oxidized in the air and thereby undergo self-combustion due to oxidation heat.
Remediation of Soil and Other Media Using Iron Powder
Polluted groundwater may bring about far more crucial damage over surface drainage, since identification of pollution sources is usually difficult in polluted groundwater as compared to polluted surface drainage. Accordingly, prompt decontamination of polluted groundwater is urgently needed. Durability of the activity of the iron powder as a reducing agent is also strongly required for using the iron powder because the iron powder cannot frequently be replaced. The organic halogen compounds may also be present as a gas in the polluted soil and air different from the organic halogen compounds in the drainage and groundwater. Therefore, it is advantageous to establish a method for efficiently decontaminating organic halogen compounds in the gas for remediation of the polluted soil and air. Accordingly, it would be advantageous to provide a method for rapidly decomposing the organic halogen compounds, a fine composite iron powder suitable for decomposition, and a method for producing the composite iron powder.
Application of Iron Powder to Magnetic Materials
It would also be advantageous to provide a composite magnetic iron powder that is a low-cost magnetic material, can exhibit its functions in the high frequency regions and is not oxidized to generate heat even in air, a method for producing the same, and an electromagnetic wave absorber using the composite magnetic iron powder.
We have discovered that reduction of an iron oxide can yield a composite iron powder including a different component and a different texture from conventional fine iron powders and that the resulting composite iron powder has advantageous properties. Specifically, this invention provides, in a first aspect, a magnetite-iron based composite powder including magnetite and iron and having an average primary particle size of from about 0.01 to about 10 xcexcm.
Preferably, the ratio of the maximum diffraction intensity of the magnetite to that of xe2x80x94-Fe in X-ray diffraction is from about 0.001 to about 50. The magnetite-iron based composite powder preferably contains at least one component selected from the group consisting of nickel, cobalt, chromium, manganese and copper, of which nickel is typically preferred. The magnetite-iron based composite powder according to the first aspect of the invention is also briefly referred to as xe2x80x9ccomposite powderxe2x80x9d.
In a second aspect, the invention provides a magnetite-iron based composite powder mixture including the magnetite-iron based composite powder and a nonferrous inorganic compound powder. The term xe2x80x9cnonferrous inorganic compoundxe2x80x9d as used herein also includes nonferrous pure elements, but does not exclude inclusion of elemental iron in the nonferrous inorganic compound in an amount as much as that of impurities (less than or equal to about 1%).
Preferably, the nonferrous inorganic compound powder has average primary particle size of less than or equal to about 0.1 xcexcm and adheres to a surface of the magnetite-iron based composite powder. In this configuration, the average primary particle size of the nonferrous inorganic compound powder should be smaller than that of the magnetite-iron based composite powder.
Alternatively, the magnetite-iron based composite powder preferably adheres to the surface of the nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm. In this configuration, the average primary particle size of the nonferrous inorganic compound powder should be greater than that of the magnetite-iron based composite powder.
Further alternatively, the magnetite-iron based composite powder and a nonferrous inorganic compound powder having an average primary particle size of less than or equal to about 0.1 xcexcm (a first nonferrous inorganic compound powder) adhere to a surface of a nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm (a second nonferrous inorganic compound powder). In this configuration, the average primary particle sizes of the first nonferrous inorganic compound powder and the magnetite-iron based composite powder are smaller than that of the second nonferrous inorganic compound powder. The magnetite-iron based composite powder mixture according to the second aspect of the invention is also briefly referred to as xe2x80x9ccomposite powder mixturexe2x80x9d.
Preferred magnetite-iron based composite powders for use in the magnetite-iron based composite powder mixture are the same as in the magnetite-iron based composite powder according to the first aspect of the invention.
Preferably, the nonferrous inorganic compound powder is a silicate and/or an inorganic compound including carbon (inclusive of allotropes of carbon). More preferably, the nonferrous inorganic compound powder having an average primary particle size of less than or equal to about 0.1 xcexcm is a silicate and/or an inorganic compound including carbon and having an average primary particle size of less than or equal to about 0.1 xcexcm, and the nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm is a silicate and/or graphite having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm.
The nonferrous inorganic compound powder having an average primary particle size of less than or equal to about 0.1 xcexcm may be referred to as an xe2x80x9cultrafine powderxe2x80x9d, and the nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm may be referred to as xe2x80x9csmall particlesxe2x80x9d.
When special emphasis is placed on the magnetic properties of the composite powder mixture, the nonferrous inorganic compound powder (inclusive of the ultrafine powder and small particles) is a dielectric powder having a relative dielectric constant of more than about 2.0.
The dielectric powder preferably has a standard Gibbs free energy of formation less than that of hematite.
Preferably, the dielectric powder is at least one material selected from the group consisting of a titanium oxide powder, a silicon oxide powder and an aluminium oxide powder.
In a third aspect, the invention provides a method for producing a magnetite-iron based composite powder. The method includes the steps of heating and thereby reducing a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas, and stopping reduction of the powder at about midstream of reduction to thereby yield a partially reduced powder as a composite powder comprising magnetite and iron.
In a fourth aspect, the invention provides another method for producing a magnetite-iron composite powder mixture. This method includes the steps of heating and thereby reducing a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas in the presence of a nonferrous inorganic compound powder, and stopping reduction of the powder at about midstream of reduction to thereby yield a partially reduced powder as a composite powder comprising magnetite and iron.
The phrase xe2x80x9cstopping reduction of the powder at about midstream of reductionxe2x80x9d means that the reduction operation of the hematite based powder is stopped during formation of water formed as a result of reduction.
The invention further provides, in a fifth aspect, another method for producing a magnetite-iron composite powder. The method includes the steps of heating a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas to reduce the powder substantially completely, and oxidizing a surface of the substantially completely reduced powder with an oxygen-containing gas to thereby yield a composite powder comprising magnetite and iron.
The invention provides, in a sixth aspect, another method for producing a magnetite-iron composite powder mixture. The method includes the steps of heating a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas in the presence of a nonferrous inorganic compound powder to reduce the powder substantially completely, and oxidizing a surface of the substantially completely reduced powder with an oxygen-containing gas to thereby yield a composite powder comprising magnetite and iron.
In a seventh aspect, the invention provides yet another method for producing a magnetite-iron based composite powder. The method includes the steps of heating and thereby reducing a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas, stopping reduction of the powder at about midstream of reduction to yield a partially reduced powder, and oxidizing a surface of the partially reduced powder with an oxygen-containing gas to thereby yield a composite powder comprising magnetite and iron.
In addition, the invention provides, in an eighth aspect, yet another method for producing a magnetite-iron composite powder mixture. The method includes the steps of heating and thereby reducing a hematite based powder having an average primary particle size of from about 0.01 to about 10 xcexcm in a reducing gas in the presence of a nonferrous inorganic compound powder, stopping reduction of the powder at about midstream of reduction to yield a partially reduced powder, and oxidizing a surface of the partially reduced powder with an oxygen-containing gas to thereby yield a composite powder comprising magnetite and iron.
According to the third through eighth aspects of the invention, the ratio of the maximum diffraction intensity of the magnetite to that of xcex1-Fe in X-ray diffraction is preferably from about 0.001 to about 50 as in the first and second aspects of the invention. The reducing gas is preferably hydrogen gas, carbon monoxide gas or a gaseous mixture thereof. Further, the reducing gas may comprise a gas of hydrocarbon such as methane or ethane.
The magnetite-iron based composite powder mentioned in the third to sixth aspects of the invention preferably contains at least one component selected from the group consisting of nickel, cobalt, chromium, manganese and copper, of which nickel is typically preferred, as in the first and second aspects of the invention.
In the above methods according to the fourth, sixth and eighth aspects of the invention, the nonferrous inorganic compound powder preferably includes a silicate and/or an inorganic compound including carbon.
In addition, the nonferrous inorganic compound powder is preferably a nonferrous inorganic compound powder having an average primary particle size of less than or equal to about 0.1 xcexcm and/or a nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm. More preferably, the nonferrous inorganic compound powder having an average primary particle size of less than or equal to about 0.1 xcexcm includes a silicate and/or an inorganic compound including carbon, and the nonferrous inorganic compound powder having an average primary particle size of equal to or more than about 1 xcexcm and less than or equal to about 100 xcexcm includes a silicate and/or an inorganic compound including carbon.
The nonferrous inorganic compound powder is preferably a dielectric powder having a relative dielectric constant of more than about 2.0.
The dielectric mentioned above preferably has a standard Gibbs free energy of formation less than that of the iron oxide.
At least one of a titanium oxide powder, a silicon oxide powder and an aluminium oxide powder is preferably used as the dielectric powder.
The material hematite powder preferably contains at least one component selected from the group consisting of nickel, cobalt, chromium, manganese and copper, of which nickel is typically preferred.
The reducing gas is preferably hydrogen gas or carbon monoxide gas.
The invention provides, in a ninth aspect, a method for remedying polluted soil, water or gases. The method includes the steps of bringing the magnetite-iron based composite powder according to the first aspect or the magnetite-iron based composite powder mixture according to the second aspect of the invention into contact with at least one of soil, water or a gas polluted with an organic halogen compound, and thereby decomposing the organic halogen compound.
In addition and advantageously, the invention provides, in a tenth aspect, an electromagnetic wave absorber including a molded mixture of the magnetite-iron based composite powder according to the first aspect or the magnetite-iron based composite powder mixture according to the second aspect with a rubber and/or a resin.
The resin is preferably a thermosetting resin or a thermoplastic resin.
The composite powder (mixture) of the invention has a larger specific surface area and more active sites than conventional iron powders for use in dehalogenation of organic halogen compounds. Accordingly, the composite powder (mixture) can rapidly dehalogenate the organic halogen compounds, keep its activity over a long time and is, therefore, suitable for remediation of polluted soil, polluted groundwater or polluted air. The composite powder (mixture) can also be used as a magnetic material for use in high frequency regions, and the resulting magnetic material obtained by molding the composite powder (mixture) can keep its satisfactory permeability and absorption capability of electromagnetic waves in the high frequency regions.